1. Field of the Invention
This invention relates to a new method for measuring fracture toughness of a somewhat non-elastic specimen that is smaller in size than has heretofore been required to perform a valid K.sub.Ic test.
2. Prior Art
It is characteristic of our times that the strength of materials from which the machines of our technology are formed are becoming ever more critical in machine design. Particularly with regard to such things as the drilling of deep wells to tap the hydrocarbon resources of the globe, the materials from which drill bits and the cutting surfaces therefore has become increasingly critical to the tools' operations. Such metals must withstand the great temperatures, pressures, and stresses underground and afford a driller with a maximum life to minimize the cost and time involved in pulling a drill string to replace a bit. Other examples of a clear and present need for providing accurate and useful materials testing data are readily apparent in the aerospace industry, and the like.
For the past two decades, various apparatus and techniques have been proposed and developed for determining the fracture toughness characteristics of ductile and brittle materials. The present invention builds upon this technology to provide a simplified technique for measuring fracture toughness of sizes of specimens that heretofore were not appropriate for such testing procedures. Specifically, the present invention builds on an earlier patent by the present inventor entitled, "Method For Measuring Plane Strength Fracture Toughness", U.S. Pat. No. 4,116,049, issued Sept. 25, 1978. As with the method of this cited patent, the present invention depends upon keeping a steady state crack tip configuration and utilizes a maximum plane strain loading whereat, at a known or measured location, a critical fracture will occur useful in computation of the K.sub.Ic for such specimen. The present invention, distinct therefrom however, is not restricted to larger specimens as called for in ASTM E 399-74, but rather can produce accurate test results for elastic-plastic specimens that are much smaller in size than would formerly have been required for a performance of a valid ASTM K.sup.Ic measurement. Specifically, the present invention provides for obtaining an accurate fracture toughness K.sub.Ic measurement of certain sizes of specimens that heretofore were appropriate for elastic-plastic fracture parameter testing based on a J integral method, or J.sub.Ic, only, which test procedure is much more complicated than that of the present invention, and therefore, the method of the present invention is a significant improvement thereover.
Like the earlier cited patented method of the present inventor for measuring plane strain fracture toughness, the present method also involves calculating the stress intensity factor or fracture toughness, K.sub.Ic, mathematically considering the critical load applied to the specimen, the specimen size and geometry, optionally, takes into account Poisson's ratio for the specimen material, and does not require comparison of test results with tests of standard specimens, or the like. Distinct from my earlier process, the present method can be used on smaller specimens than heretofore were appropriate as the test results therefrom do not depend on the specimen exhibiting perfectly elastic characteristics. Rather, the present method allows and provides for the occurrence of plasticity or anelastic behavior around the tip of the crack in the specimen. Plasticity, of course, is the drawback to prior fracture toughness testing methods as, particularly with small specimens, it cannot generally be ignored.
The present method, unique from the cited prior patent method, involves obtaining a steady stage fracture after initial crack growth and employs at least two unloading sequences or cycles whereby, after the fracture is initiated and has progressed through or past a critical point, the specimen loading is removed, and the specimen relaxes along a straight load-displacement line. Thereafter, by re-loading the specimen and again relaxing it, after further crack extension, an area under the crack extension curve, between load-displacement lines, is produced indicative of the work done to advance the crack between the two points. This data, along with measurements of specimen compliance and taking into account specimen geometry, can be used to derive the specimen fracture toughness, K.sub.Ic. This derivation, where the specimen does not exhibit perfect elasticity, allows for anelastic behavior in the specimen that prohibits a return of the specimen sides to their original configuration when the load is removed.
The present method, similar to the earlier patented method of the present inventor, provides a simplified and yet accurate technique for measuring fracture toughness of specimens and, as will be elaborated on later herein, has been used in a laboratory setting to produce acceptable measurements of 6061-T651 aluminum alloy specimens having only a 12.7 millimeter diameter, a much smaller diameter of specimen than could formerly by used to produce a valid K.sub.Ic test by my earlier patented method.
Within the knowledge of the inventor, the method of the present invention is unlike any heretofore known and is a significant improvement in the art as it provides for a simplified testing technique for use on specimens that have not heretofore been proper subjects to test for plane strain fracture toughness, and the present method is therefore believed to be both novel and unique and a significant improvement in the art.